1. Field of the Invention
The present invention relates to an improvement in a still image-taking device (camera) having a focal plane shutter and a strobe light emission device.
2. Related Background Art
In a still image-taking device having the focal plane shutter, a slit exposure is conducted in a fast shutter speed having no time where the focal plane shutter is fully opened, i.e., in a so-called high-speed shutter mode.
FIG. 7 is a diagram showing a relationship between travel states of a leading shutter curtain and a trailing shutter curtain and an exposure time at a slit exposure time, in which the leading shutter curtain starts to travel at a time t11, and an exposure starts from an upper end of the image-taking element. Then, the trailing shutter curtain starts to travel at a time t12 delayed from t11 by a given period of time, in which light-shielding is made from the upper end of the image taking element to stop the exposure. The travel speeds of the leading shutter curtain and the trailing shutter curtain are so adjusted as to be identical with each other, and the exposure is sequentially conducted over an overall region of the upper end to the lower end of the image taking element for a given time period Td (t12 to t11). Times t13 and t14 are travel completion times of the leading shutter curtain and the trailing shutter curtain, and a time period (t14 to t13) is also a given time period Td.
When strobe image taking operation is going to be conducted during the above slit exposure, since the strobe light emission can be conducted only during the exposure of a part of a screen as shown in FIG. 8, resulting in an exposure unevenness, the strobe image taking operation is conducted only in a slow shutter speed having a time where the focal plane shutter is fully opened up to now. However, in recent years, a strobe light emission device having a so-called flat light emission mode that emits a light having a constant light emission intensity for a long period of time as compared with the conventional device is produced, and strobe image taking operation can be conducted even at the time of a high speed shutter operation which involves the slit exposure by using this mode.
FIG. 9 shows the operation of the leading shutter curtain and the trailing shutter curtain and the light emission state of the strobe light emission device when the image taking operation with the strobe is conducted during the slit exposure by using the strobe light emission device having the flat light emission mode.
The operation will be described with reference to FIG. 9. The strobe light emission device starts to emit a light at a time t10 prior to a time t11 at which the leading shutter curtain starts to travel. In this situation, an interval between the time t10 and the time t11 is set to a given period of time which is necessary to stabilize the light emission intensity of the strobe light emission device. That is, after the light emission intensity of the strobe light emission device has been stabilized, the leading shutter curtain starts to travel, and then the trailing shutter curtain starts to travel at a time t12 after a given time period Td has elapsed. In addition, when the travel of the trailing shutter curtain is completed at a time t14, the light emission of the strobe light emission device is stopped. The image taking operation with the strobe can be thus conducted even at the high-speed shutter mode having no shutter full open time.
However, in the above-mentioned still image-taking device having the focal plane shutter, although the strobe light emission device emits a light with a constant intensity from the travel start of the leading shutter curtain till the travel completion of the trailing shutter curtain in the strobe image taking operation at the high speed shutter mode, the strobe light reaches only a slight part of the image taking element if attention is paid to a time point during the exposure, and the remaining strobe light is absorbed by the shutter curtain. For example, in FIG. 9, assuming that a given time period Td which corresponds to an exposure period of time (shutter speed) is 1 msec, and the travel period of time of the leading shutter curtain and the trailing shutter curtain (t13-t11)=(t14-t12) is 4 msec, the flat light emission period of time during which a stabilized strobe light is emitted as required is at least 5 msec, and an energy of 80% or more is in vain. Also, because a light emission intensity must be restricted in order to conduct the flat light emission for a long period of time such as 5 msec, there are such disadvantages that an energy loss is large and a guide number indicative of the effective distance of the strobe image taking operation is also small.